A thing is when 2 people like each other, and they're "seeing each other" and no one else. And if they do shit together no one will be called a slut or anything cause they have a thing... the urge of dating but not official

Guy 1: I have a thing for this girl
Guy 2: So that's what you've been up to the last 3 weeks.
Guy 1: Yeah, it's the only thing i do.

An action, fashion style, philosophy, musical genre, or other popularly recognized subsection of popular culture. Normally used in surprise at it's existence. Becomes official when a wikipedia article is created for it.

"Whats the deal with those skin-tight body suits?"
"Oh, you mean Zentai?
"Huh. I didn't know that was a thing."